Magnetic tape, magnetic tape cartridge, and magnetic tape apparatus

ABSTRACT

The magnetic tape includes a non-magnetic support; a magnetic layer including ferromagnetic powder and a binding agent on one surface side of the non-magnetic support; and a back coating layer including non-magnetic powder and a binding agent on the other surface side of the non-magnetic support, in which an isoelectric point of a surface zeta potential of the magnetic layer is equal to or smaller than 3.8, and an isoelectric point of a surface zeta potential of the back coating layer is equal to or smaller than 3.0, a magnetic tape cartridge, and a magnetic tape apparatus including this magnetic tape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2018-141813 filed on Jul. 27, 2018. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape, a magnetic tapecartridge, and a magnetic tape apparatus.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recordingmedia and disk-shaped magnetic recording media, and tape-shaped magneticrecording media, that is, magnetic tapes are mainly used for storagesuch as data back-up.

As the magnetic tapes, a magnetic tape including a back coating layer ona surface side of a non-magnetic support opposite to a surface sideprovided with a magnetic layer has been disclosed in JP1997-190623A(JP-H09-190623A).

SUMMARY OF THE INVENTION

A magnetic tape is generally accommodated in a magnetic tape cartridgein a state of being wound around a reel. The recording of information onthe magnetic tape and the reproducing thereof are generally performed bysetting a magnetic tape cartridge in a magnetic tape apparatus called adrive, and causing the magnetic tape to run in the magnetic tapeapparatus, and causing a surface of the magnetic tape (surface of amagnetic layer) and a magnetic head to come into contact with each otherfor sliding.

The magnetic tape is used in a data center in which a temperature andhumidity are controlled. In addition, a temperature and humidity of astorage environment before shipping of the magnetic tape cartridgeaccommodating the magnetic tape are also controlled.

Meanwhile, in the data center, power saving is required for costreduction. In order to realize the power saving, it is desirable tofurther alleviate the controlling conditions of a usage environment ofthe magnetic tape in the data center than current conditions or make thecontrol unnecessary. This viewpoint applies to the storage environmentof the magnetic tape cartridge.

However, it is assumed that, in a case where the controlling conditionsof the usage environment and/or the storage environment are alleviatedor the controlling thereof is not performed, the magnetic tape isexposed to an environmental change due to a change in the weather, achange of the season, and the like. As one aspect of such anenvironmental change, a temperature change from a low temperature to ahigh temperature under high humidity is considered.

Regarding the point described above, from the studies of the inventors,it is determined that, in a magnetic tape including a back coating layeron a surface side of a non-magnetic support opposite to a surface sideprovided with a magnetic layer, in a case where the sliding between asurface of the magnetic tape and a magnetic head is repeated, after atemperature change from a low temperature to a high temperature underhigh humidity occurs, a frequency of generation of a partial decrease inreproducing signal amplitude (referred to as “missing pulse”) increases,during the reproducing of information recorded on the magnetic tape. Asthe generation frequency of the missing pulse increases, an error rateincreases and reliability of a magnetic tape is deteriorated.

One aspect of the invention provides for a magnetic tape including aback coating layer on a surface side of a non-magnetic support oppositeto a surface side provided with a magnetic layer, in which a generationfrequency of missing pulse is low, even in a case where sliding betweena surface of the magnetic tape and a magnetic head is repeated, afterthe magnetic tape is exposed to a temperature change from a lowtemperature to a high temperature under high humidity.

According to one aspect of the invention, there is provided a magnetictape comprising: a non-magnetic support; a magnetic layer includingferromagnetic powder and a binding agent on one surface side of thenon-magnetic support; and a back coating layer including non-magneticpowder and a binding agent on the other surface side of the non-magneticsupport, in which an isoelectric point of a surface zeta potential ofthe magnetic layer is equal to or smaller than 3.8, and an isoelectricpoint of a surface zeta potential of the back coating layer is equal toor smaller than 3.0.

In one aspect, the isoelectric point of a surface zeta potential of themagnetic layer may be 2.5 to 3.8.

In one aspect, the isoelectric point of a surface zeta potential of theback coating layer may be 2.0 to 3.0.

In one aspect, the binding agent of the magnetic layer may be a bindingagent including an acidic group.

In one aspect, the binding agent of the back coating layer may be abinding agent including an acidic group.

In one aspect, the acidic group may include at least one kind of acidicgroup selected from the group consisting of a sulfonic acid group and asalt thereof.

In one aspect, the magnetic tape may further comprise a non-magneticlayer including non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.

According to another aspect of the invention, there is provided amagnetic tape cartridge comprising: the magnetic tape described above.

According to still another aspect of the invention, there is provided amagnetic tape apparatus comprising: the magnetic tape described above;and a magnetic head.

According to one aspect of the invention, it is possible to provide amagnetic tape including a back coating layer on a surface side of anon-magnetic support opposite to a surface side provided with a magneticlayer, in which a generation frequency of missing pulse is low, even ina case where sliding between a surface of the magnetic tape and amagnetic head is repeated, after the magnetic tape is exposed to atemperature change from a low temperature to a high temperature underhigh humidity. In addition, according to one aspect of the invention, itis possible to provide a magnetic tape cartridge and a magnetic tapeapparatus including the magnetic tape described above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape

One aspect of the invention relates to a magnetic tape including: anon-magnetic support; a magnetic layer including ferromagnetic, powderand a binding agent on one surface side of the non-magnetic support; anda back coating layer including non-magnetic powder and a binding agenton the other surface side of the non-magnetic support, in which anisoelectric point of a surface zeta potential of the magnetic layer isequal to or smaller than 3.8, and an isoelectric point of a surface zetapotential of the back coating layer is equal to or smaller than 3.0.

Hereinafter, the magnetic tape will be described more specifically. Inthe invention and the specification, a “surface of the magnetic layer”is identical to the surface of the magnetic tape on the magnetic layerside, and a “surface of the back coating layer” is identical to thesurface of the magnetic tape on the back coating layer side.

Isoelectric Point of Surface Zeta Potential

In the magnetic tape, the isoelectric point of the surface zetapotential of the magnetic layer is equal to or smaller than 3.8, and theisoelectric point of the surface zeta potential of the back coatinglayer is equal to or smaller than 3.0. In the invention and thespecification, the isoelectric point of the surface zeta potential ofeach layer is a value of pH, in a case where a surface zeta potentialmeasured by a flow potential method (also referred to as a flow currentmethod) becomes zero. A sample is cut out from the magnetic tape whichis a measurement target, and the sample is disposed in a measurementcell so that the surface of a layer which is a target for obtaining thesurface zeta potential (surface of magnetic layer or back coating layer)comes into contact with an electrolyte. Pressure in the measurement cellis changed to flow the electrolyte and a flow potential at each pressureis measured, and then, the surface zeta potential is obtained by thefollowing calculation expression.

Calculation  Expression$\zeta = {\frac{dI}{dp} \times \frac{\eta}{{ɛɛ}_{0}}\frac{L}{A}}$

[ζ: surface zeta potential, p: pressure, I: flow potential, η: viscosityof electrolyte, ε: relative dielectric constant of electrolyte, ε₀:dielectric constant in a vacuum state, L: length of channel (flow pathbetween two electrodes), A: area of cross section of channel]

The pressure is changed in a range of 0 to 400,000 Pa (0 to 400 mbar).The calculation of the surface zeta potential by flowing the electrolyteto the measurement cell and measuring a flow potential is performed byusing electrolytes having different pH (from pH of 9 to pH of 3 atinterval of approximately 0.5). A total number of measurement points is13 from the measurement point of pH 9 to the 13th measurement points ofpH 3. By doing so, the surface zeta potential of each measurement pointof pH is obtained. As pH decreases, the surface zeta potentialdecreases. Thus, two measurement points at which polarity of the surfacezeta potential changes (a change from a positive value to a negativevalue) may appear, while pH decreases from 9 to 3. In a case where suchtwo measurement points appear, pH, in a case where the surface zetapotential is zero, is obtained by interpolation by using a straight line(linear function) showing a relationship between the surface zetapotential and pH of each of the two measurement points. Meanwhile, in acase Where all of the surface zeta potentials obtained during thedecrease of pH from 9 to 3 is positive value, pH, in a case where thesurface zeta potential is zero, is obtained by extrapolation by using astraight line (linear function) showing a relationship between thesurface zeta potential and pH of the 13th measurement point (pH of 3)which is the final measurement point and the 12th measurement point. Onthe other hand, in a case where all of the surface zeta potentialsobtained during the decrease of pH from 9 to 3 is negative value, pH, ina case where the surface zeta potential is zero, is obtained byextrapolation by using a straight line (linear function) showing arelationship between the surface zeta potential and pH of the firstmeasurement point (pH of 9) which is the initial measurement point andthe 12th measurement point. By doing so, the value of pH, in a casewhere the surface zeta potential of the magnetic layer measured by theflow potential method is zero, is obtained.

The above measurement is performed three times in total at roomtemperature by using different samples cut out from the same magnetictape (magnetic tape which is a measurement target), and pH, in a casewhere the surface zeta potential in each measurement is zero, isobtained. For the viscosity and the relative dielectric constant of theelectrolyte, a measurement value at room temperature is used. The roomtemperature is set as 20° C. to 27° C. Regarding the magnetic layer, anarithmetical mean of three pHs obtained as described above is anisoelectric point of the surface zeta potential of the magnetic layer ofthe magnetic tape which is a measurement target. In addition, regardingthe back coating layer, an arithmetical mean of three pHs obtained asdescribed above is an isoelectric point of the surface zeta potential ofthe back coating layer of the magnetic tape which is a measurementtarget. As the electrolyte having pH of 9, an electrolyte obtained byadjusting pH of a KCl aqueous solution having a concentration of 1mmol/L to 9 by using a KOH aqueous solution having a concentration of0.1 mol/L is used. As the electrolyte having other pH, an electrolyteobtained by adjusting pH of the electrolyte having pH of 9, which isadjusted as described above, by using an HCl aqueous solution having aconcentration of 0.1 mol/L is used.

The isoelectric point of the surface zeta potential measured by themethod described above is an isoelectric point obtained regarding thesurface of the magnetic layer or the back Coating layer. As a result ofthe intensive studies, the inventors have newly found that, by settingthe isoelectric point of the surface zeta potential of the magneticlayer to be equal to or smaller than 3.8 and setting the isoelectricpoint of the surface zeta potential of the back coating layer to beequal to or smaller than 3.0, it is possible to decrease a generationfrequency of a missing pulse due to a temperature change from a lowtemperature to a high temperature under high humidity. In regards tothis point, the inventors have surmised as follows. However, thefollowing description is merely a surmise, and the invention is notlimited thereto.

Scraps (may be referred to as debris) may be generated due to chippingof the surface of the magnetic layer during the sliding between thesurface of the magnetic tape (surface of the magnetic layer) and themagnetic head. The inventors have surmised that, in a case where themagnetic tape is exposed to a temperature change from a low temperatureto a high temperature under high humidity, a component of the magneticlayer is precipitated on the surface of the magnetic layer, therebyeasily generating a precipitate on the surface of the magnetic layer. Itis thought that, in a case where the scraps and/or the precipitate isstrongly stuck to the surface of the magnetic layer, a contact statebetween the surface of the magnetic layer and the magnetic head is notstabilized, thereby easily generating a partial decrease of areproducing signal amplitude (missing pulse). With respect to this, theinventors have thought that, in a case of using the magnetic layer in aregion of acidic pH in which the isoelectric point of the surface zetapotential is equal to or smaller than 3.8, a repulsive force may beeasily applied between the scraps and/or precipitate and the surface ofthe magnetic layer. The inventors have surmised that preventing thescraps and/or precipitate from being strongly stuck to the surface ofthe magnetic layer by this repulsive force may contribute to a decreasein the generation frequency of the missing pulse.

In addition, the inventors have surmised that, in a case where themagnetic tape is exposed to a temperature change from a low temperatureto a high temperature under high humidity, a component of the backcoating layer is precipitated, thereby easily generating a precipitateon the surface of the back coating layer. It is thought that, in a casewhere this precipitate is strongly stuck to the surface of the backcoating layer, stability of a running state of the magnetic tape isdeteriorated. Due to this reason, it is also surmised that the missingpulse is easily generated. With respect to this, the inventors havethought that, in a case of using the back coating layer in a region ofacidic pH in which the isoelectric point of the surface zeta potentialis equal to or smaller than 3.0, a repulsive force may be easily appliedbetween the precipitate and the surface of the back coating layer. Theinventors have surmised that preventing the precipitate from beingstrongly stuck to the surface of the back coating layer by thisrepulsive force may contribute to a decrease in the generation frequencyof the missing pulse.

However, the invention is not limited to the above surmise and othersurmises described in the specification.

From a viewpoint of further decreasing the generation frequency of themissing pulse, the isoelectric point of the surface zeta potential ofthe magnetic layer is preferably equal to or smaller than 3.7, morepreferably equal to or smaller than 3.6, even more preferably equal toor smaller than 3.5, still preferably equal to or smaller than 3.4, andstill more preferably equal to or smaller than 3.3.

As will be described later in detail, the isoelectric point of thesurface zeta potential of the magnetic layer can be controlled by thekind of a component used for forming the magnetic layer, a formationstep of the magnetic layer, and the like. From a viewpoint ofavailability of a component (for example, binding agent) used forforming the magnetic layer, the isoelectric point of the surface zetapotential of the magnetic layer is preferably equal to or greater than2.0, more preferably equal to or greater than 2.5, even more preferablyequal to or greater than 2.6, and still preferably equal to or greaterthan 2.7.

From a viewpoint of further decreasing the generation frequency of themissing pulse, the isoelectric point of the surface zeta potential ofthe back coating layer is preferably equal to or smaller than 2.9, morepreferably equal to or smaller than 2.8, even more preferably equal toor smaller than 2.7, and still more preferably equal to or smaller than2.6, and still even more preferably equal to or smaller than 2.5.

As will be described later in detail, the isoelectric point of thesurface zeta potential of the back coating layer can be controlled bythe kind of a component used for forming the back coating layer, aformation step of the back coating layer, and the like. From a viewpointof availability of a component (for example, binding agent) used forforming the back coating layer, the isoelectric point of the surfacezeta potential of the back coating layer is preferably equal to orgreater than 1.5, more preferably equal to or greater than 1.8, and evenmore preferably equal to or greater than 2.0.

Next, the magnetic layer, the back coating layer, and the like of themagnetic tape will be described more specifically.

Magnetic Layer

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic tape. Fromthis viewpoint, ferromagnetic powder having an average particle sizeequal to or smaller than 50 nm is preferably used, and ferromagneticpowder having an average particle size equal to or smaller than 40 nm ismore preferably used, as the ferromagnetic powder. Meanwhile, theaverage particle size of the ferromagnetic powder is preferably equal toor greater than 5 nm, more preferably equal to or greater than 10 nm,even more preferably equal to or greater than 15 nm, and stillpreferably equal to or greater than 20 nm, from a viewpoint of stabilityof magnetization.

As a preferred specific example of the ferromagnetic powder, hexagonalferrite powder can be used. For details of the hexagonal ferrite powder,descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A,paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 ofJP2012-204726A, and paragraphs 0029 to 0084 of JP2015-127985A can bereferred to, for example.

As a preferred specific example of the ferromagnetic powder, metalpowder can also be used. For details of the metal powder, descriptionsdisclosed in paragraphs 0137 to 0141 of JP2011-216149A and paragraphs0009 to 0023 of JP2005-251351A can be referred to, for example.

As a preferable specific example of the ferromagnetic powder, ε-ironoxide powder can also be used. As a manufacturing method of the ε-ironoxide powder, a manufacturing method from a goethite, a reverse micellemethod, and the like are known. All of the manufacturing methods arewell known. In addition, regarding a method of manufacturing the ε-ironoxide powder in which a part of Fe is substituted with substitutionalatoms such as Ga, Co, Ti, Al, or Rh, a description disclosed in J. Jpn.Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280 to S284, J.Mater. Chem. C, 2013, 1, pp. 5200 to 5206 can be referred, for example.However, the manufacturing method of the ε-iron oxide powder capable ofbeing used as the ferromagnetic powder in the magnetic layer is notlimited.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so as to have the total magnification of 500,000 toobtain an image of particles configuring the powder. A target particleis selected from the obtained image of particles, an outline of theparticle is traced with a digitizer, and a size of the particle (primaryparticle) is measured. The primary particle is an independent particlewhich is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of the plurality ofparticles not only includes an aspect in which particles configuring theaggregate are directly in contact with each other, but also includes anaspect in which a binding agent or an additive which will be describedlater is interposed between the particles. A term “particles” is alsoused for describing the powder.

As a method of collecting a sample powder from the magnetic tape inorder to measure the particle size, a method disclosed in a paragraph0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, and in a case of the definition (2), the average particle sizeis an average plate diameter. In a case of the definition (3), theaverage particle size is an average diameter (also referred to as anaverage particle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. The components other than the ferromagnetic powder ofthe magnetic layer are at least a binding agent and one or more kinds ofadditives may be further randomly included. A high filling percentage ofthe ferromagnetic powder in the magnetic layer is preferable from aviewpoint of improvement recording density.

Binding Agent and Curing Agent

The magnetic tape is a coating type magnetic tape and includes a bindingagent in the magnetic layer. The binding agent is one or more kinds ofresin. As the binding agent, various resins normally used as a bindingagent of a coating type magnetic recording medium can be used. Forexample, as the binding agent, a resin selected from a polyurethaneresin, a polyester resin, a polyamide resin, a vinyl chloride resin, anacrylic resin obtained by copolymerizing styrene, acrylonitrile, ormethyl methacrylate, a cellulose resin such as nitrocellulose, an epoxyresin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinylacetal or polyvinyl butyral can be used alone or a plurality of resinscan be mixed with each other to be used. Among these, a polyurethaneresin, an acrylic resin, a cellulose resin, and a vinyl chloride resinare preferable. These resins may be homopolymers or copolymers. Theseresins can be used as the binding agent even in the non-magnetic layerand/or a back coating layer which will be described later.

For the binding agent described above, description disclosed inparagraphs 0028 to 0031 of JP2010-024113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 to 200,000 as a weight-average molecular weight. Theweight-average molecular weight of the invention and the specificationis a value obtained by performing polystyrene conversion of a valuemeasured by gel permeation chromatography (GPC) under the followingmeasurement conditions. The weight-average molecular weight of thebinding agent shown in examples which will be described later is a valueobtained by performing polystyrene conversion of a value measured underthe following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THE.)

In one aspect, as the binding agent, a binding agent including an acidicgroup can be used. The acidic group of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and a salt thereof. Specific examples of the acidicgroup include a sulfonic acid group, a sulfate group, a carboxyl group,a phosphate group, and a salt thereof. For example, a salt of sulfonicacid group (—SO₃H) is represented by —SO₃M, and M represents a grouprepresenting an atom (for example, alkali metal atom or the like) whichmay be cations in water or in an aqueous solvent. The same applies toaspects of salt of various groups described above. As an example of thebinding agent including the acidic group, a resin including at least onekind of acidic group selected from the group consisting of a sulfonicacid group and a salt thereof (for example, a polyurethane resin or avinyl chloride resin) can be used. However, the resin included in themagnetic layer is not limited to these resins. In addition, in thebinding agent including the acidic group, a content of the acidic groupcan be, for example, 0.03 to 0.50 meq/g. The content of variousfunctional groups such as the acidic group included in the resin can beobtained by a well-known method in accordance with the kind of thefunctional group. The amount of the binding agent used in a magneticlayer forming composition can be, for example, 1.0 to 30.0 parts by masswith respect to 100.0 parts by mass of the ferromagnetic powder.

In regards to the controlling of the isoelectric point of the surfacezeta potential of the magnetic layer, the inventors have surmised thatformation of the magnetic layer so that the acidic component is unevenlydistributed to a surface portion of the magnetic layer contributes to adecrease in value of the isoelectric point. In addition, it is surmisedthat formation of the magnetic layer so as to decrease the amount of abasic component present in the surface portion of the magnetic layeralso contributes to a decrease in value of the isoelectric point. Theacidic component is used as a meaning including a state of a componentcapable of emitting H⁺ in water or an aqueous solvent to dissociateanions and a salt thereof. The basic component is used as a meaningincluding a state of a component capable of emitting OH⁻ in water or anaqueous solvent to dissociate cations and a salt thereof. For example,it is thought that, in a case of using the acidic component, unevendistribution of the acidic component to the surface portion of themagnetic layer contributes to a decrease in value of the isoelectricpoint of the surface zeta potential of the magnetic layer to control theisoelectric point to be equal to or smaller than 3.8. For example, it isthought that, in a step of applying a magnetic layer forming compositiononto a non-magnetic support directly or through a non-magnetic layer,the applying which is performed in an alternating magnetic field byapplying an alternating magnetic field contributes to formation of amagnetic layer in which the acidic component is unevenly distributed tothe surface portion. As the acidic component, for example, a bindingagent including an acidic group can be used. In addition, it is surmisedthat, in a case of using the binding agent including an acidic group, ina preparation step of the magnetic layer forming composition, theaddition (additional addition) of the binding agent even in a case ofpreparing a dispersion liquid (magnetic liquid) including ferromagneticpowder and the binding agent and then mixing the magnetic liquid withother components, contributes to formation of a magnetic layer in whichthe binding agent including the acidic group is unevenly distributed tothe surface portion. The formation of the magnetic layer will bedescribed later more specifically.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent is included in the magneticlayer in a state of being reacted (crosslinked) with other componentssuch as the binding agent, by proceeding the curing reaction in themagnetic layer forming step. This point is the same as regarding a layerformed by using a composition, in a ease where the composition used forforming the other layer includes the curing agent. The preferred curingagent is a thermosetting compound, polyisocyanate is suitable. Fordetails of the polyisocyanate, descriptions disclosed in paragraphs 0124and 0125 of JP2011-216149A can be referred to, for example. The amountof the curing agent can be, for example, 0 to 80.0 parts by mass withrespect to 100.0 parts by mass of the binding agent in the magneticlayer forming composition, and is preferably 50.0 to 80.0 parts by mass,from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and the binding agent,and may include one or more kinds of additives, if necessary. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive included in the magnetic layerinclude non-magnetic powder (for example, inorganic powder or carbonblack), a lubricant, a dispersing agent, a dispersing assistant, anantibacterial agent, an antistatic agent, and an antioxidant. As thenon-magnetic powder, non-magnetic powder which can function as anabrasive, non-magnetic powder (for example, non-magnetic colloidparticles) which can function as a projection formation agent whichforms projections suitably protruded from the surface of the magneticlayer, and the like can be used. An average particle size of colloidalsilica (silica colloid particles) shown in the examples which will bedescribed later is a value obtained by a method disclosed in ameasurement method of an average particle diameter in a paragraph 0015of JP2011-048878A. As the additives, a commercially available productcan be suitably selected according to the desired properties ormanufactured by a well-known method, and can be used with any amount. Asan example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive. For example, for thelubricant, descriptions disclosed in paragraphs 0030 to 0033, 0035, and0036 of JP2016-126817A can be referred to. The non-magnetic layer mayinclude a lubricant. For the lubricant which may be included in thenon-magnetic layer, descriptions disclosed in paragraphs 0030, 0031,0034, 0035, and 0036 of JP2016-126817A can be referred to. For thedispersing agent, a description disclosed in paragraphs 0061 and 0071 ofJP2012-133837A can be referred to. The dispersing agent may be includedin the non-magnetic layer. For the dispersing agent Which can beincluded in the non-magnetic layer, a description disclosed in aparagraph 0061 of JP2012-133837A can be referred to.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Back Coating Layer

The back coating layer at least includes non-magnetic powder and abinding agent. As the non-magnetic powder included in the back coatinglayer, any one or both of carbon black and non-magnetic powder otherthan carbon black can be used. As the non-magnetic powder other thancarbon black, powder of an inorganic substance (inorganic powder) can beused. Specific examples thereof include inorganic powder of iron oxidesuch as α-iron oxide, titanium oxide such as titanium dioxide, ceriumoxide, tin oxide, tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂O₃, α-alumina,β-alumina, γ-alumina, goethite, corundum, silicon nitride, titaniumcarbide, magnesium oxide, boron nitride, molybdenum disulfide, copperoxide, MgCO₃, CaCO₃, BaCO₃, SrCO₃, BaSO₄, and silicon carbide. For thenon-magnetic powder included in the back coating layer, the descriptionregarding the non-magnetic powder included in the non-magnetic layerwhich will be described later can also be referred to.

A shape of the non-magnetic powder other than carbon black may be any ofa needle shape, a sphere shape, a polyhedron shape, and a planar shape.An average particle size of the non-magnetic powder is preferably 0.01to 0.20 μm and more preferably 0.01 to 0.15 μm. In addition, a specificsurface area of the non-magnetic powder obtained by aBrunauer-Emmett-Teller (BET) method (BET specific surface area) ispreferably 1 to 100 m²/g, more preferably 5 to 70 m²/g, and even morepreferably 10 to 65 m²/g. Meanwhile, an average particle size of carbonblack is, for example, 5 to 80 nm, preferably 10 to 50 nm, and morepreferably 10 to 40 nm. For the content (filling percentage) of thenon-magnetic powder in the back coating layer, the description regardingthe non-magnetic powder of the non-magnetic layer which will bedescribed later can be referred to.

The back coating layer further includes a binding agent and can randomlyinclude well-known additives. The amount of the binding agent used in aback coating layer forming composition can be, for example, 1.0 to 30.0parts by mass with respect to 100.0 parts by mass of the non-magneticpowder.

In one aspect, as the binding agent of the back coating layer, a bindingagent including an acidic group can be used. For details of the bindingagent including the acidic group, the above description can be referredto.

For other details of the binding agent, additives, and the like of theback coating layer, a well-known technology regarding the back coatinglayer can be applied, and a well-known technology regarding the magneticlayer and/or the non-magnetic layer can be applied. For example, for theback coating layer, descriptions disclosed in paragraphs 0018 to 0020 ofJP2006-331625A and page 4, line 65, to page 5, line 38, of U.S. Pat. No.7,029,774B can be referred to.

In regards to the controlling of the isoelectric point of the surfacezeta potential of the back coating layer, the inventors have surmisedthat formation of the back coating layer so that the acidic component isunevenly distributed to a surface portion of the back coating layercontributes to a decrease in value of the isoelectric point. Inaddition, it is surmised that formation of the hack coating layer so asto decrease the amount of a basic component present in the surfaceportion of the back coating layer also contributes to a decrease invalue of the isoelectric point. For example, it is thought that, in acase of using the acidic component, uneven distribution of the acidiccomponent to the surface portion of the back coating layer contributesto a decrease in value of the isoelectric point of the surface zetapotential of the back coating layer to control the isoelectric point tobe equal to or smaller than 3.0. For example, the inventors have thoughtthat, in a step of applying a back coating layer forming compositiononto a non-magnetic support, the applying which is performed in analternating magnetic field by applying an alternating magnetic fieldcontributes to formation of a back coating layer in which the acidiccomponent is unevenly distributed to the surface portion. As the acidiccomponent, for example, a binding agent including an acidic group can beused. In addition, the inventors have surmised that, in a case of usingthe binding agent including an acidic group, in a preparation step ofthe back coating layer forming composition, the addition (additionaladdition) of the binding agent even in a case of preparing a dispersionliquid including non-magnetic powder and the binding agent and thenmixing this dispersion liquid with other components, contributes toformation of a back coating layer in which the binding agent includingthe acidic group is unevenly distributed to the surface portion. Theformation of the back coating layer will be described later morespecifically.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape mayinclude a magnetic layer directly on the surface of the non-magneticsupport or may include a magnetic layer on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer including the non-magnetic powder and the binding agent. Thenon-magnetic powder used in the non-magnetic layer may be inorganicpowder or organic powder. In addition, carbon black and the like can beused. Examples of the inorganic powder include powder of metal, metaloxide, metal carbonate, metal sulfate, metal nitride, metal carbide, andmetal sulfide. These non-magnetic powder can be purchased as acommercially available product or can be manufactured by a well-knownmethod. For details thereof, descriptions disclosed in paragraphs 0146to 0150 of JP2011-216149A can be referred to. For carbon black which canbe used in the non-magnetic layer, descriptions disclosed in paragraphs0040 and 0041 of JP2010-024113A can be referred to. The content (fillingpercentage) of the non-magnetic powder of the non-magnetic layer ispreferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

In the invention and the specification, the non-magnetic layer alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to asa “support”) will be described. As the non-magnetic support, well-knowncomponents such as polyethylene terephthalate, polyethylene naphthalate,polyamide, polyamide imide, and aromatic polyamide subjected to biaxialstretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heat treatmentmay be performed with respect to these supports in advance.

Various Thicknesses

A thickness of the non-magnetic support is preferably 3.00 to 20.00 μm,more preferably 3.00 to 10.00 μm, and even more preferably 3.00 to 6.00μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount of a magnetic head used, a head gaplength, a recording signal band, and the like. The thickness of themagnetic layer is normally 0.01 μm to 0.15 μm, and is preferably 0.02 μmto 0.12 μm and more preferably 0.03 μm to 0.10 μm from a viewpoint ofrealization of high-density recording. The magnetic layer may be atleast one layer, or the magnetic layer can be separated into two or morelayers having different magnetic properties, and a configurationregarding a well-known multilayered magnetic layer can be applied. Athickness of the magnetic layer which is separated into two or morelayers is a total thickness of these layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand is preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and more preferably 0.10 to 0.70 μm.

The thicknesses of various layers of the magnetic tape and thenon-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magnetic tapein a thickness direction is, for example, exposed by a well-known methodof ion beams or microtome, and the exposed cross section is observedwith a scanning electron microscope. In the cross section observation,various thicknesses can be acquired as a thickness acquired at oneportion of the cross section, or an arithmetical mean of thicknessesacquired at a plurality of portions of two or more portions, forexample, two portions which are randomly extracted. In addition, thethickness of each layer may be acquired as a designed thicknesscalculated according to the manufacturing conditions.

Manufacturing Method of Magnetic Tape

Each composition for forming the magnetic layer, the back coating layer,and the non-magnetic layer which is randomly provided, normally includesa solvent, together with various components described above. As thesolvent, various organic solvents generally used for manufacturing acoating type magnetic recording medium can be used. The amount of thesolvent in each layer forming composition is not particularly limited,and can be set to be the same as that of each layer forming compositionof a typical coating type magnetic recording medium. Steps of preparingthe composition for forming each layer can generally include at least akneading step, a dispersing step, and a mixing step provided before andafter these steps, if necessary. Each step may be divided into two ormore stages. The components used in the preparation of each layerforming composition may be added at an initial stage or in a middlestage of any step. In addition, each component may be separately addedin two or more steps. For example, in the preparation of the magneticlayer forming composition, the binding agent including an acidic groupcan be separately added through two or more steps. It is preferable thata dispersion liquid is prepared by mixing some components including theferromagnetic powder among the various components of the magnetic layerforming composition with the binding agent including an acidic group anddispersing the mixture in a solvent, and the binding agent including anacidic group is also added in a step of mixing the dispersion liquidwith the remaining components and performing dispersing, because it ispossible to contribute to the controlling of the isoelectric point of asurface zeta potential of the magnetic layer to be equal to or smallerthan 3.8. In addition, it is preferable that the non-magnetic powderwhich can function as an abrasive is dispersed separately from theferromagnetic powder, and then other components such as theferromagnetic powder are mixed and dispersed, in order to improvedispersibility of the ferromagnetic powder and the non-magnetic powder(abrasive). For example, in the preparation of the back coating layerforming composition, the binding agent including an acidic group can beseparately added through two or more steps. It is preferable that adispersion liquid is prepared by mixing some components including thenon-magnetic powder among the various components of the back coatinglayer forming composition with the binding agent including an acidicgroup and dispersing the mixture in a solvent, and the binding agentincluding an acidic group is also added in a step of mixing thedispersion liquid with the remaining components and performingdispersing, because it is possible to contribute to the controlling ofthe isoelectric point of a surface zeta potential of the back coatinglayer to be equal to or smaller than 3.0.

In order to prepare each layer forming composition, a well-knowntechnology can be used. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. The details ofthe kneading processes of these kneaders are disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-079274A (JP-H01-079274A). In addition, inorder to disperse each layer forming composition, as a dispersionmedium, at least one or more kinds of dispersion beads selected from thegroup consisting of glass beads and other dispersion beads can be used.As such dispersion beads, zirconia beads, titania beads, and steel beadswhich are dispersion beads having high specific gravity are suitable.These dispersion beads can be used by optimizing a particle diameter(head diameter) and a filling percentage. As a disperser, a well-knowndisperser can be used. Each layer forming composition may be filtered bya well-known method before performing the coating step. The filteringcan be performed by using a filter, for example. As the filter used inthe filtering, a filter having a hole diameter of 0.01 to 3 μm (forexample, filter made of glass fiber or filter made of polypropylene) canbe used, for example.

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the surface of the non-magnetic support orperforming multilayer coating with the non-magnetic layer formingcomposition in order or at the same time. The back coating layer can beformed by applying the back coating layer forming composition onto thesurface of the non-magnetic support where the magnetic layer is formedor the surface thereof on a side opposite to the surface where themagnetic layer is to be formed. For details of the coating for formingeach layer, a description disclosed in a paragraph 0066 ofJP2010-231843A can be referred to.

The coating of the magnetic layer forming composition performed in analternating magnetic field can contribute to the controlling of theisoelectric point of a surface zeta potential of the magnetic layer tobe equal to or smaller than 3.8. It is surmised that this is because, anacidic component (for example, the binding agent including an acidicgroup) is easily unevenly distributed to a surface portion of a coatinglayer of the magnetic layer forming composition due to the appliedalternating magnetic field, and thus, by drying this coating layer, amagnetic layer in which the acidic component is unevenly distributed tothe surface portion is obtained. The applying of the alternatingmagnetic field can be performed by disposing a magnet in a coatingdevice so that the alternating magnetic field is applied vertically tothe surface of the coating layer of the magnetic layer formingcomposition. A magnetic field strength of the alternating magnetic fieldcan be, for example, set as approximately 0.05 to 3.00 T. However, thereis no limitation to this range.

In addition, the coating of the back coating layer forming compositionperformed in an alternating magnetic field can contribute to thecontrolling of the isoelectric point of a surface zeta potential of theback coating layer to be equal to or smaller than 3.0. It is surmisedthat this is because, an acidic component (for example, the bindingagent including an acidic group) is easily unevenly distributed to asurface portion of a coating layer of the back coating layer formingcomposition due to the applied alternating magnetic field, and thus, bydrying this coating layer, a back coating layer in which the acidiccomponent is unevenly distributed to the surface portion is obtained.The applying of the alternating magnetic field can be performed bydisposing a magnet in a coating device so that the alternating magneticfield is applied vertically to the surface of the coating layer of theback coating layer forming composition. A magnetic field strength of thealternating magnetic field can be, for example, set as approximately0.05 to 3.00 T. However, there is no limitation to this range.

The “vertical” in the invention and the specification does not mean onlya vertical direction in the strict sense, but also includes a range oferrors allowed in the technical field of the invention. For example, therange of errors means a range of less than ±10° from an exact verticaldirection.

For various other steps for manufacturing the magnetic tape, awell-known technology can be applied. For details of the various steps,descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A canbe referred to, for example. It is preferable that the coating layer ofthe magnetic layer forming composition is subjected to an alignmentprocess, while this coating layer is wet (not dried). For the alignmentprocess, various well-known technologies such as a description disclosedin a paragraph 0067 of JP2010-231843A can be used. For example, ahomeotropic alignment process can be performed by a well-known methodsuch as a method using a polar opposing magnet. In an alignment zone, itis possible to control a drying speed of the coating layer by atemperature of dry air, an air flow, and/or a transportation speed ofthe magnetic tape in the alignment zone. In addition, the coating layermay be preliminarily dried before being transported to the alignmentzone. In a case of performing the alignment process, it is preferable toapply a magnetic field (for example, DC magnetic field) for aligning theferromagnetic powder with respect to the coating layer of the magneticlayer forming composition applied in the alternating magnetic field.

As described above, it is possible to obtain the magnetic tape accordingto one aspect of the invention. The magnetic tape is normallyaccommodated in a magnetic tape cartridge and the magnetic tapecartridge is mounted in a magnetic tape apparatus (generally referred toas a “drive”). A servo pattern can also be formed on the magnetic layerof the magnetic tape by a well-known method, in order to allow headtracking servo to be performed in the drive.

According to the magnetic tape, even in a case where the sliding betweenthe surface of the magnetic tape and the magnetic head is repeated,after the magnetic tape is exposed to a temperature change from a lowtemperature to a high temperature under high humidity, it is possible todecrease the generation frequency of the missing pulse. In one aspect,high humidity can be relative humidity of 70% to 100%, a low temperaturecan be higher than 0° C. and equal to or lower than 15° C., a hightemperature can be 30° C. to 50° C., and a temperature change from a lowtemperature to a high temperature can be a temperature changeapproximately from 15° C. to 50° C.

Magnetic Tape Cartridge

One aspect of the invention relates to a magnetic tape cartridgeincluding the magnetic tape.

In the magnetic tape cartridge, the magnetic tape is generallyaccommodated in a cartridge main body in a state of being wound around areel. The reel is rotatably provided in the cartridge main body. As themagnetic tape cartridge, a single reel type magnetic tape cartridgeincluding one reel in a cartridge main body and a twin reel typemagnetic tape cartridge including two reels in a cartridge main body arewidely used. In a case where the single reel type magnetic tapecartridge is mounted in the magnetic tape apparatus (drive) in order torecord and/or reproduce information (magnetic signals) on the magnetictape, the magnetic tape is drawn from the magnetic tape cartridge andwound around the reel on the drive side. A magnetic head is disposed ona magnetic tape transportation path from the magnetic tape cartridge toa winding reel. Sending and winding of the magnetic tape are performedbetween a reel (supply reel) on the magnetic tape cartridge side and areel (winding reel) on the drive side. In the meantime, the magnetichead comes into contact with and slides on the surface of the magneticlayer of the magnetic tape, and accordingly, the recording and/orreproduction of information is performed. With respect to this, in thetwin reel type magnetic tape cartridge, both reels of the supply reeland the winding reel are provided in the magnetic tape cartridge. Themagnetic tape cartridge may be any of single reel type magnetic tapecartridge and twin reel type magnetic tape cartridge. The magnetic tapecartridge may include the magnetic tape according to one aspect of theinvention, and a well-known technology can be applied for otherconfigurations.

Magnetic Tape Apparatus

One aspect of the invention relates to a magnetic tape apparatusincluding the magnetic tape and a magnetic head.

In the invention and the specification, the “magnetic tape apparatus”means a device capable of performing at least one of the recording ofinformation on the magnetic tape or the reproducing of informationrecorded on the magnetic tape. Such an apparatus is generally called adrive. The magnetic tape apparatus can be a sliding type magnetic tapeapparatus. The sliding type apparatus is an apparatus in which thesurface of the magnetic layer comes into contact with and slides on themagnetic head, in a case of performing the recording of information onthe magnetic tape and/or reproducing of the recorded information.

The magnetic head included in the magnetic tape apparatus can be arecording head capable of performing the recording of information on themagnetic tape, or can be a reproducing head capable of performing thereproducing of information recorded on the magnetic tape. In addition,in one aspect, the magnetic tape apparatus can include both of arecording head and a reproducing head as separate magnetic heads. Inanother aspect, the magnetic head included in the magnetic tape can alsohave a configuration of comprising both of a recording element and areproducing element in one magnetic head. As the reproducing head, amagnetic head (MR head) including a magnetoresistive (MR) elementcapable of sensitively reading information recorded on the magnetic tapeas a reproducing element is preferable. As the MR head, variouswell-known MR heads can be used. In addition, the magnetic head whichperforms the recording of information and/or the reproducing ofinformation may include a servo pattern reading element. Alternatively,as a head other than the magnetic head which performs the recording ofinformation and/or the reproducing of information, a magnetic head(servo head) comprising a servo pattern reading element may be includedin the magnetic tape apparatus.

In the magnetic tape apparatus, the recording of information on themagnetic tape and/or the reproducing of information recorded on themagnetic tape can be performed by bringing the surface of the magneticlayer of the magnetic tape into contact with the magnetic head andsliding. The magnetic tape apparatus may include the magnetic tapeaccording to one aspect of the invention and well-known technologies canbe applied for other configurations.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and “%by mass”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted.

A “binding agent A” described below is a SO₃Na group-containingpolyurethane resin (weight-average molecular weight: 70,000, SO₃Nagroup: 0.20 meq/g).

A “binding agent B” described below is a vinyl chloride copolymer(product name: MR110, SO₃K group-containing vinyl chloride copolymer,SO₃K group: 0.07 meq/g) manufactured by Kaneka Corporation.

Manufacturing of Magnetic Tape

Example 1

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a SO₃Na group-containingpolyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g)manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent ofmethyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solventwere mixed with 100.0 parts of alumina powder (HIT-80 manufactured bySumitomo Chemical Co., Ltd.) having a gelatinization ratio ofapproximately 65% and a BET specific surface area of 20 m²/g, anddispersed in the presence of zirconia beads by a paint shaker for 5hours. After the dispersion, the dispersion liquid and the beads wereseparated by a mesh and an alumina dispersion was obtained.

(2) Magnetic Layer Forming Composition List

Magnetic Liquid Ferromagnetic powder 100.0 parts Hexagonal bariumferrite powder having average particle size (average plate diameter) of21 nm Binding agent (see Table 1) see Table 1 Cyclohexanone 150.0 partsMethyl ethyl ketone 150.0 parts Abrasive Solution Alumina dispersionprepared in the section (1) 6.0 parts Silica Sol (projection formingagent liquid) Colloidal silica (Average particle size: 120 nm) 2.0 partsMethyl ethyl ketone 1.4 parts Other Components Stearic acid 2.0 partsStearic acid amide 0.2 parts Butyl stearate 2.0 parts Polyisocyanate(CORONATE (registered trademark) 2.5 parts manufactured by TosohCorporation) Finishing Additive Solvent Cyclohexanone 200.0 parts Methylethyl ketone 200.0 parts

(3) Non-Magnetic Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide 100.0 parts Average particlesize (average long axis length): 0.15 μm Average acicular ratio: 7 BETspecific surface area: 52 m²/g Carbon black 20.0 parts Average particlesize: 20 nm Binding agent A 18.0 parts Stearic acid 2.0 parts Stearicacid amide 0.2 parts Butyl stearate 2.0 parts Cyclohexanone 300.0 partsMethyl ethyl ketone: 300.0 parts

(4) Back Coating Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide 80.0 parts Average particlesize (average long axis length): 0.15 μm Average acicular ratio: 7 BETspecific surface area: 52 m²/g Carbon black: 20.0 parts Average particlesize: 20 nm Binding agent (see Table 1) see Table 1 Phenylphosphonicacid 3.0 parts Methyl ethyl ketone 155.0 parts Polyisocyanate 5.0 partsCyclohexanone 355.0 parts

(5) Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

The magnetic liquid was prepared by dispersing (beads-dispersing)various components described above by using a batch type vertical sandmill for 24 hours. Zirconia beads having a bead diameter of 0.5 mm wereused as the dispersion beads.

The prepared magnetic liquid, the abrasive solution, the binding agent Aadditionally added (0.1 parts with respect to 100.0 parts of theferromagnetic powder of the magnetic liquid), and other components(silica sol, other components, and finishing additive solvent) weremixed with each other and beads-dispersed for 5 minutes by using thesand mill, and the treatment (ultrasonic dispersion) was performed witha batch type ultrasonic device (20 kHz, 300 W) for 0.5 minutes. Afterthat, the obtained mixed solution was filtered by using a filter havinga hole diameter of 0.5 μm, and the magnetic layer forming compositionwas prepared.

The non-magnetic layer forming composition was prepared by the followingmethod.

Various components described above excluding the lubricant (stearicacid, stearic acid amide, and butyl stearate), cyclohexanone, and methylethyl ketone were dispersed by using batch type vertical sand mill for24 hours to obtain a dispersion liquid. As the dispersion beads,zirconia beads having a bead diameter of 0.5 mm were used. After that,the remaining components were added into the obtained dispersion liquidand stirred with a dissolver. The dispersion liquid obtained asdescribed above was filtered with a filter having a hole diameter of 0.5μm and a non-magnetic layer forming composition was prepared.

The back coating layer forming composition was prepared by the followingmethod.

Various components described above excluding polyisocyanate andcyclohexanone were kneaded by an open kneader and diluted, and weresubjected to a dispersion process of 12 passes, with a transverse beadsmill disperser and zirconia beads having a bead diameter of 1 mm, bysetting a bead filling percentage as 80 volume %, a circumferentialspeed of rotor distal end as 10 msec, and a retention time for 1 pass as2 minutes. After that, the remaining components and the binding agent Ato be additionally added (0.1 parts with respect to 100.0 parts of thenon-magnetic powder (non-magnetic inorganic powder and carbon black))were added into the obtained dispersion liquid and stirred with adissolver. The dispersion liquid obtained as described above wasfiltered with a filter having a hole diameter of 1 μm and a back coatinglayer forming composition was prepared.

(6) Manufacturing Method of Magnetic Tape

The non-magnetic layer forming composition prepared in the section (5)was applied to a surface of a support made of polyethylene naphthalatehaving a thickness of 5.00 μm so that the thickness after the dryingbecomes 1.00 μm and was dried to form a non-magnetic layer.

Then, in a coating device disposed with a magnet for applying analternating magnetic field, the magnetic layer forming compositionprepared in the section (5) was applied onto the surface of thenon-magnetic layer so that the thickness after the drying becomes 0.10μm, while applying an alternating magnetic field (magnetic fieldstrength: 0.15 T), to form a coating layer. The applying of thealternating magnetic field was performed so that the alternatingmagnetic field was applied vertically to the surface of the coatinglayer. After that, a homeotropic alignment process was performed byapplying a magnetic field having a DC magnetic field strength of 0.30 Tin a vertical direction with respect to a surface of a coating layer,while the coating layer of the magnetic layer forming composition is wet(not dried). After that, the coating layer was dried to form a magneticlayer.

After that, the back coating layer forming composition prepared in thesection (5) was applied to the surface of the support made ofpolyethylene naphthalate on a side opposite to the surface where thenon-magnetic layer and the magnetic layer were formed, so that thethickness after the drying becomes 0.50 μm, and was dried to form a backcoating layer. The coating of the back coating layer forming compositionwas performed while applying the alternating magnetic field (magneticfield strength: 0.15 T) vertically to the surface of the coating layerof the back coating layer forming composition, in a coating devicedisposed with a magnet for applying the alternating magnetic field.

After that, a surface smoothing treatment (calender process) wasperformed by using a calender roll configured of only a metal roll, at aspeed of 100 m/min, linear pressure of 294 kN/m (300 kg/cm), and acalender temperature (surface temperature of a calender roll) of 100° C.

Then, the heat treatment was performed in the environment of theatmosphere temperature of 70° C. for 36 hours. After the heat treatment,the slitting was performed to have a width of ½ inches (1 inch is 0.0254meters), and a servo pattern was formed on the magnetic layer by acommercially available servo writer.

By doing so, a magnetic tape of Example 1 was manufactured.

Examples 2 to 5 and Comparative Examples 1 to 10

A magnetic tape was manufactured by the same method as in Example 1,except that various conditions were changed as shown in Table 1.

In Table 1, in the examples and the comparative examples in which“performed” is shown in the column of the additional addition of thebinding agent, the additional addition of the binding agent A wasperformed during the preparation of the magnetic layer formingcomposition and/or the back coating layer forming composition, in thesame manner as in Example 1. On the other hand, in the comparativeexamples in which “not performed” is shown in this column, theadditional addition of the binding agent A was not performed during thepreparation of the magnetic layer forming composition and/or the backcoating layer forming composition.

In Table 1, in the examples and the comparative examples in which“performed” is shown in the column of the alternating magnetic fieldapplication during coating, the step subsequent to the coating step ofthe magnetic layer forming composition and/or the back coating layerforming composition was performed by the same method as in Example 1.That is, the application of the alternating magnetic field was performedduring coating of the magnetic layer forming composition and/or the backcoating layer forming composition in the same manner as in Example 1. Onthe other hand, in the comparative examples in which “not performed” isshown in this column, the step subsequent to the coating step of themagnetic layer forming composition and/or the back coating layer formingcomposition was performed by the same method as in Example 1, exceptthat the application of the alternating magnetic field was not performedduring the coating of the magnetic layer forming composition and/or theback coating layer Ruining composition.

Evaluation of Physical Properties of Magnetic Tape

(1) Isoelectric Point of Surface Zeta Potential of Magnetic Layer

Six samples for isoelectric point measurement were cut out from eachmagnetic tape of the examples and the comparative examples and disposedin the measurement cell of two samples in one measurement. In themeasurement cell, a sample installing surface and a surface of the backcoating layer of the sample were bonded to each other by using adouble-sided tape in upper and lower sample table (size of each sampleinstalling surface is 1 cm×2 cm) of the measurement cell. In a casewhere an electrolyte flows in the measurement cell after disposing twosamples as described above, the surface of the magnetic layer of the twosamples bonded to each other on the upper and lower sample table of themeasurement cell comes into contact with the electrolyte, and thus, thesurface zeta potential of the magnetic layer can be measured. Themeasurement was performed three times in total by using two samples ineach measurement, and the isoelectric points of the surface zetapotential of the magnetic layer were obtained. An arithmetical mean ofthe obtained three values obtained by three times of the measurement isshown in Table 1, as the isoelectric point of the surface zeta potentialof the magnetic layer of each magnetic tape. As a surface zeta potentialmeasurement device, SurPASS manufactured by Anton Paar was used. Themeasurement conditions were set as follows. Other details of the methodof obtaining the isoelectric point are as described above.

Measurement cell: variable gap cell (20 mm×10 mm)

Measurement mode: Streaming Current

Gap: approximately 200 μm

Measurement temperature: room temperature

Ramp Target Pressure/Time: 400,000 Pa (400 mbar)/60 seconds

Electrolyte: KCl aqueous solution having concentration of 1 mmol/L(adjusted pH to 9)

pH adjusting solution: HCl aqueous solution having concentration of 0.1mol/L or KOH aqueous solution having concentration of 0.1 mol/L

Measurement pH: pH 9→pH 3 (measured at 13 measurement points in total atinterval of approximately 0.5)

(2) Isoelectric Point of Surface Zeta Potential of Back Coating Layer

Six samples for isoelectric point measurement were cut out from eachmagnetic tape of the examples and the comparative examples and disposedin the measurement cell of two samples in one measurement. In themeasurement cell, a sample installing surface and a surface of themagnetic layer of the sample were bonded to each other by using adouble-sided tape in upper and lower sample table (size of each sampleinstalling surface is 1 cm×2 cm) of the measurement cell. In a casewhere an electrolyte flows in the measurement cell after disposing twosamples as described above, the surface of the back coating layer of thetwo samples bonded to each other on the upper and lower sample table ofthe measurement cell comes into contact with the electrolyte, and thus,the surface zeta potential of the back coating layer can be measured.The measurement was performed three times in total by using two samplesin each measurement, and the isoelectric points of the surface zetapotential of the back coating layer were obtained. An arithmetical meanof the obtained three values obtained by three times of the measurementis shown in Table 1, as the isoelectric point of the surface zetapotential of the back coating layer of each magnetic tape. As a surfacezeta potential measurement device, the device disclosed in the section(1) was used, and the measurement conditions were set as shown in thesection (1). Other details of the method of obtaining the isoelectricpoint are as described above.

(3) Missing Pulse Generation Frequency

A magnetic tape cartridge accommodating each magnetic tape (magnetictape total length of 500 m) of the examples and the comparative exampleswas stored in a thermo box in which a temperature was maintained to be10° C. and relative humidity was maintained to be 80%, for 3 hours.After that, the magnetic tape cartridge was extracted from the thermobox (a temperature of the outside air was 23° C. and relative humiditywas 50%), and put in a thermo room in which a temperature was maintainedto be 32° C. and relative humidity was maintained to be 80%, within 1minute, and set in a drive of Linear Tape-Open Generation 6 (LTO-G6)manufactured by IBM in this thermo room. After that, in the drive, themagnetic tape in the magnetic tape cartridge was subjected toreciprocating running 1,500 times at tension of 0.6 N and a runningspeed of 8 m/sec, while bringing the magnetic head of the drive intocontact with the surface of the magnetic layer for sliding.

In the thermo room, the magnetic tape cartridge after the running wasextracted from the drive, and set in another drive (LTO-G6 drivemanufactured by IBM), the magnetic tape was caused to run, and therecording and reproducing of information were performed, while bringingthe magnetic head into contact with the surface of the magnetic layerfor sliding. A reproducing signal during the running was introduced toan external analog/digital (AD) conversion device. A signal having areproducing signal amplitude which is decreased 70% or more than anaverage (average of measured values at each track) was set as a missingpulse, a generation frequency (number of times of the generation)thereof was divided by the total length of the magnetic tape to obtain amissing pulse generation frequency (unit: times/m) per unit length ofthe magnetic tape (per 1 m), In a case where the missing pulsegeneration frequency is equal to or smaller than 5.0 number/m, themagnetic tape can be determined as a magnetic tape having highreliability in practice.

The results of the above evaluation are shown in Table 1 (Table 1-1 andTable 1-2).

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Formation ofmagnetic layer Content of binding agent in Binding agent A 5.0 parts10.0 parts 15.0 parts 20.0 parts 10.0 parts magnetic liquid Bindingagent B   0 parts   0 parts   0 parts   0 parts 10.0 parts Additionaladdition of binding agent Performed Performed Performed PerformedPerformed Alternating magnetic field application during PerformedPerformed Performed Performed Performed coating Formation of backcoating Content of binding agent in Binding agent A 5.0 parts 10.0 parts15.0 parts 20.0 parts 10.0 parts layer dispersion liquid Binding agent B  0 parts   0 parts   0 parts   0 parts 10.0 parts Additional additionof binding agent Performed Performed Performed Performed PerformedAlternating magnetic field application during Performed PerformedPerformed Performed Performed coating Isoelectric point of surface zetapotential of magnetic layer 3.8 3.5 3.0 2.5 3.0 Isoelectric point ofsurface zeta potential of back coating layer 3.0 2.8 2.5 2.0 2.7 Missingpulse generation frequency (times/m) 4.0 3.5 3.5 3.0 3.5 Compar- Compar-Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- ativeative ative ative ative ative ative ative ative ative Example Example 1Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8Example 9 10 Formation Content Binding 5.0 parts 10.0 parts 15.0 parts20.0 parts 15.0 parts 15.0 parts 0 parts 10.0 parts 10.0 parts 10.0parts of of agent A magnetic binding Binding   0 parts   0 parts   0parts   0 parts   0 parts   0 parts   10 parts   10 parts   0 parts   0parts layer agent in agent B mag- netic liquid Additional Not Not NotNot Performed Not Not Not Performed Not addition of performed performedperformed performed performed performed performed performed bindingagent Alternating Not Not Not Not Not Performed Not Not Performed Notmagnetic field performed performed performed performed performedperformed performed performed application during coating FormationContent Binding 5.0 parts 10.0 parts 15.0 parts 20.0 parts 15.0 parts15.0 parts   0 parts 10.0 parts 10.0 parts 10.0 parts of back of agent Acoating binding Binding   0 parts   0 parts   0 parts   0 parts   0parts   0 parts 10.0 parts 10.0 parts   0 parts   0 parts layer agent inagent B dis- persion liquid Additional Not Not Not Not Performed Not NotNot Not Performed addition of performed performed performed performedperformed performed performed performed binding agent Alternating NotNot Not Not Not Performed Not Not Not Performed magnetic field performedperformed performed performed performed performed performed performedapplication during coating Isoelectric point of surface 5.0 4.8 4.6 4.64.2 4.6 4.8 4.7 3.6 4.8 zeta potential of magnetic layer Isoelectricpoint of surface 4.0 3.8 3.7 3.7 4.0 4.0 4.0 4.0 3.8 2.7 zeta potentialof back coating layer Missing pulse generation 25.0 15.0 15.0 14.0 18.018.0 15.0 15.0 8.0 8.0 frequency (times/m)

From the results shown in Table 1, in the magnetic tapes of theexamples, it is possible to confirm that the missing pulse generationfrequency is lower than those of the magnetic tapes of the comparativeexamples, even after the magnetic tape was exposed to a temperaturechange from a low temperature to a high temperature under high humidity.

One aspect of the invention is effective in the technical fields ofvarious magnetic tapes for data storage.

What is claimed is:
 1. A magnetic tape comprising: a non-magneticsupport; a magnetic layer including ferromagnetic powder and a bindingagent on one surface side of the non-magnetic support; and a backcoating layer including non-magnetic powder and a binding agent on theother surface side of the non-magnetic support, wherein an isoelectricpoint of a surface zeta potential of the magnetic layer is equal to orsmaller than 3.8, and an isoelectric point of a surface zeta potentialof the back coating layer is equal to or smaller than 3.0.
 2. Themagnetic tape according to claim 1, wherein the isoelectric point of asurface zeta potential of the magnetic layer is 2.5 to 3.8.
 3. Themagnetic tape according to claim 1, wherein the isoelectric point of asurface zeta potential of the back coating aver is 2.0 to 3.0.
 4. Themagnetic tape according to claim 1, wherein the binding agent of themagnetic layer is a binding agent including an acidic group.
 5. Themagnetic tape according to claim 4, wherein the acidic group includes atleast one kind of acidic group selected from the group consisting of asulfonic acid group and a salt thereof.
 6. The magnetic tape accordingto claim 2, wherein the binding agent of the magnetic layer is a bindingagent including an acidic group.
 7. The magnetic tape according to claim6, wherein the acidic group includes at least one kind of acidic groupselected from the group consisting of a sulfonic acid group and a saltthereof.
 8. The magnetic tape according to claim 3, wherein the bindingagent of the magnetic layer is a binding agent including an acidicgroup.
 9. The magnetic tape according to claim 8, wherein the acidicgroup includes at least one kind of acidic group selected from the groupconsisting of a sulfonic acid group and a salt thereof.
 10. The magnetictape according to claim 1, wherein the binding agent of the back coatinglayer is a binding agent including an acidic group.
 11. The magnetictape according to claim 10, wherein the acidic group includes at leastone kind of acidic group selected from the group consisting of asulfonic acid group and a salt thereof.
 12. The magnetic tape accordingto claim 2, wherein the binding agent of the back coating layer is abinding agent including an acidic group.
 13. The magnetic tape accordingto claim 12, wherein the acidic group includes at least one kind ofacidic group selected from the group consisting of a sulfonic acid groupand a salt thereof.
 14. The magnetic tape according to claim 3, whereinthe binding agent of the back coating layer a binding agent including anacidic group.
 15. The magnetic tape according to claim 14, wherein theacidic group includes at least one kind of acidic group selected fromthe group consisting of a sulfonic acid group and a salt thereof. 16.The magnetic tape according to claim 4, wherein the binding agent of theback coating layer is a binding agent including an acidic group.
 17. Themagnetic tape according to claim 16, wherein the acidic group includesat least one kind of acidic group selected from the group consisting ofa sulfonic acid group and a salt thereof.
 18. The magnetic tapeaccording to claim 1, further comprising: a non-magnetic layer includingnon-magnetic powder and a binding agent between the non-magnetic supportand the magnetic layer.
 19. A magnetic tape cartridge comprising: themagnetic tape according to claim
 1. 20. A magnetic tape apparatuscomprising: the magnetic tape according to claim 1; and a magnetic head.